1. Field of the Invention
This invention relates to a resin composition providing fast curing and being distinguished in crack resistance when used according to the resin transfer molding (which will be referred to as RTM or R-RIM in the following) which is one of the molding methods of fiber reinforced thermosetting plastics (which will be referred to a FRP in the following).
2. Prior Art
RTM features in that it permits low pressure, low temperature molding so that the equipment investment such as mold and press costs can be reduced. However, it has difficulties in productivity and moldability.
To satisfy these requirements, a resin composition distinguished in curing performance or of fast curing and in resistance to cracking is required. A such resin composition, an unsaturated polyester resin composition is cited in EP-A-234692 laid open on Jan. 13, 1987. It is a polymeric composition comprising an unsaturated polyester, styrene and/or acrylate monomer and a compound having a (meth)acrylate group. However, such composition was intended for improvement of the odor of the unreacted, volatile unsaturated monomer (such as styrene) in the molded product and had thus difficulties in that the compound having the (meth)acrylate group was of a low molecular weight comprising no hydroxyl group so that if the curing time (time from addition of a hardener to maximum heat generation for hardening) were accelerated, the gelling time (time from addition of the hardener to loss of fluidity) would be extremely reduced, resulting in gelation occurring during the process of injection or cracking occurring in the molded product. To prevent cracking, a thermoplastic resin such as poly(vinyl acetate) or polystyrene was blended to the unsaturated monomer solution of an unsaturated polyester, or the content of the unsaturated acid in the unsaturated polyester was reduced, for adjustment of the concentration of cross-links. However, prevention of cracking with such method had the curing time extremely extended or the mechanical strength of the molded product greatly reduced. Thus, both productivity and moldability were hardly satisfied simultaneously.